The present invention generally relates to an adaptive startup control method (ASCM) and, more specifically, to an ASCM for electric drives applicable to force-cooled electric power electronics.
The power electronics for aerospace applications plays a significant role in the modern aircraft and spacecraft industry. This is particularly true in the area of more electric architecture (MEA) for aircraft and military ground vehicles.
The commercial aircraft business is moving toward non-bleed air environmental control systems (ECS), variable-frequency (VF) power distribution systems and electrical actuation. Typical examples include the latest designs, such as the Boeing 7E7 and the Airbus jumbo A380. The next-generation Boeing airplane, Y1 (replacement of 737), and the Airbus airplane A1 (replacement of A320), will most likely use MEA. Some military aircraft already utilize MEA, including primary and secondary flight control. Military ground vehicles have migrated toward hybrid electric technology where the main propulsion is electric drives. Therefore substantial demand for power utilization has arisen.
Resulting from these tendencies is a significant increase in power conversion needs. Non-bleed ECS's need additional electric drives for vapor cycle system (VCS) compressors, condenser fans and liquid pumps. A large number of electric drives for fans are required. In constant-frequency applications, these fans have used predominately direct drive (no power electronics) to an induction machine. In the new environment, a double power electronics conversion AC to DC and DC to AC is required. Auxiliary power unit (APU) and main engine electric start imposes a need for high-power, multiple-use controllers. Military aircraft require high-voltage (270-Vdc) power conversions multiple times. This is from generation, to electric flight controllers and utilization. Future Combat Systems (FCS) have moved toward high voltage power distribution system where high-power propulsion and generation are being used.
In this environment, it is obvious that there is a need for power converters and motor controllers for aircraft and ground military businesses for increased power levels conversion capabilities to handle increased loads; reduced controller weights to be able to accommodate large content increase per platform; reduced volume to accommodate electronics housings in limited compartments space; increased reliability for achieving reasonable mission success; and reduced cost for affordability.
The power range for power conversion and motor control units varies from hundreds of watts to hundreds of kilowatts. The efficiency of these converters varies from 80 to 97%. Therefore, heat rejection from 3 to 20% of the total converted power is required. For power conversion levels above several kilowatts, forced cooling is typically needed to achieve acceptable power density levels. The forced cooling is either air or liquid. The proper utilization of the coolant flow is achieved by using special devices called cold plates both for liquids and for air.
Electric drives as a part of an ECS, or those in close proximity with an ECS, can benefit from using low-temperature coolants. However, the low temperature coolant may not become available immediately after starting up the system. It typically takes several minutes until the fluid temperature reduces to its steady-state level. At the same time, the electric drives are expected to start operation quickly at full or reduced load conditions. This is particularly valid for the vapor cycle system (VCS) compressor controller, since the ECS readiness and coolant availability are dependent on the compressor speed.
The highest temperature levels of the semiconductor devices of the power electronics will usually occur during a worst-case startup consisting of a non-operating soak to high temperatures without coolant. This may occur, for example, while an aircraft is on the ground with high (55-85° C.) surrounding temperatures. Upon startup, the coolant flow starts immediately, but the coolant temperature could still be very high. Therefore, a temperature spike would occur during initial startup of the system.
Referring to FIG. 1, there is shown a conventional motor control scheme 100 that can be realized by either analog electronics or high performance digital signal processor (DSP). High end products will generally use a sensorless control option to reduce system cost and improve reliability, while analog and digital hardwired controllers are attractive when software expenses are unacceptable.
An outer speed loop 102 may be wrapped around an inner current/torque loop 104 and the controller may be allowed to operate at full limits until the inverter baseplate temperature exceeds a predetermined value, at which time the controller is disabled as shown at 106. This type of design allows a maximum speed command without regard to the environmental conditions of the controller until loss of coolant or excessive duty cycle causes the power inverter baseplate temperature 108 to exceed the safe value.
Most controllers have a full-power operational mode, after a “hot soak” period (that is, a period of time in which the coolant is allowed to take on the temperature of its surroundings), which, in general, produces the worst-case operation thermally for most of the active power components. Continuous operation of the controller will bring a gradual cool-down of the cooling medium to the steady-state condition. Controllers designed for this type of operation will generally use their “thermal inertia (mass)” to limit the maximum junction temperatures of the active devices to a safe value. The thermal inertia is generally proportional to the weight (i.e. mass) of the object. For many applications, the need to minimize weight may be an important consideration.
Due to its high power density, the isolated gate bipolar transistor (IGBT) module is designed to closely follow the temperature of its mounting surface (usually the cold plate carrying the coolant). The IGBT device and its included junctions have a relatively short thermal time constant relative to the several minutes required for the coolant to reach its normal steady-state temperature after being “soaked” to a higher non-operating temperature environment.
Referring now to FIG. 2, there is shown a representation of the thermal curves for a coldplate-mounted IGBT module installed on a high-performance, air-cooled coldplate. After soaking to 85° C., the module is loaded at 100 percent power while the coolant temperature ramps from its initial 85° to 9° C. over a two-minute period. The low thermal inertia IGBT junctions 110 jump to a steady-state temperature above the IGBT case 112 and continue to rise as the component case 112 and heat sink 114 absorbs the dissipated power. When the coolant temperature 116 reduces sufficiently to handle the total power, component and junction temperatures 110 peak and then fall to their designed steady-state temperature limits 118. For the case shown in FIG. 2, the peak IGBT junction temperature exceeds the 150° C. rating for the component, which is unacceptable.
A more expensive, heavier, or more complex thermal design would be necessary to address the over temperature shown in FIG. 2. The design is more than adequate for steady-state operation and even hot startups provided the “soak” temperature is about 20° C. less extreme that the 85° C. used in this example.
As can be seen, there is a need for a new adaptive startup control method for electric drives with improved performance and reduced cost. This new adaptive startup control method should minimize the startup time of the system while, at the same time, maintain the semiconductor temperatures at a safe level.